Greenwich is the home of the prime meridian, the zero line from which longitude East and West are measured. (A touristy thing to do is stand with one foot in each of the Eastern and Western hemispheres.) It is also home of Greenwich mean time (GMT), and there is still a telescope in the Royal observatory, on the prime meridian, to observe when the sun hits noon. Greenwich defines the exact center of one of the 24 time zones in the world. The International Date line is arranged so that when it is between noon and 1 o'clock pm at Greenwich, all time zones in the world agree what day it is.
The Maritime museum at Greenwich has a marvellous array of clocks, a visit is highly recommended. A history of the relatively recent technology of timekeeping is on display. Many many clocks of all types can be seen - the very first (almost)pocket-sized watches, with remarkable accuracy for their era and size, grandfather clocks, sundials, even a whimsical mechanical clock that uses ballbearings and seesaws. Go see it.
Pronunciation: The first half rhymes with 'men', the second sounds like 'itch'. The double 'e' is actually a short sound, and the 'w' is not voiced.